Rainwater Garden Demonstration Projects
The City of Bloomington has partnered with the Nine Mile Creek Watershed District to build six residential rainwater gardens on Thomas Avenue in 2009, twenty two in the neighborhood just north of Old Shakopee Road and west of Nine Mile Creek in 2010, and eight more in neighborhoods near 91st & Queen Ave. S. and 83rd & Garfield Ave. The rainwater gardens are part of an on-going demonstration promoting clean water, while adding wildlife habitat, beauty and recharging groundwater. Construction of the gardens has been in conjunction with the City's Pavement Management Program (PMP) Street Reconstruction Projects.
The first project took place in 2009, 25 residents on Thomas Avenue, between 106th and 108th Streets, were contacted about voluntarily participating in Bloomington's first residential Rainwater Garden Demonstration, six households elected to participate. Barr Engineering assisted the Engineering Division in the design and construction oversight of the gardens. Barr's Landscape Architect assisted in personalized plant selection and placement to promote infiltration performance and individual flower and native grass coloring and characteristics. Due to poor weather in October, construction was put back later into fall and the planting schedule moved into November. On a sunny fall day, a small group of enthusiastic residents, Engineering staff and Barr's Landscape expert planted over 240 plants in the six gardens in a couple hours. After planting, mulch was added to prevent spring weed growth. The first growing season was a great success as there was 100% plant survival and the during the second season the plants were really filling out the gardens. A slow drive down this block in midsummer is a joy to those who enjoy colorful flowers and whispery native grasses. The Watershed District even hosted several bus tours full of statewide water resource professional through the neighborhood.
To expand on 2009's success, the Engineering Division stepped up the size of the demonstration neighborhood to a possible 100 residents, of which 22 volunteered to participate. The streets include Washburn, Vincent, Upton and Thomas Avenues between Old Shakopee Road and Bailiff Place. A special meeting was held during the PMP public information process informing the residents of the benefits of rainwater gardens and the opportunity to participate in another rainwater garden demonstration project during the 2010 street reconstruction project. A typical good yard for a rainwater garden has a gentle slope, is not covered by a large tree canopy, has generally sandy soils (typical of this whole neighborhood) and is occupied by a resident with a tendency towards gardening. With resident input and approval, engineering staff prepared individual rainwater garden designs, tailored to each of the residents' yards and drainage areas. Planting of the gardens was done in late October; early snow prevented applying the mulch until spring 2011. Plant survival was decent and the first year showed a variety of garden styles throughout the neighborhood.
Rainwater gardens are typically designed to handle twelve inches of water, and are designed to capture one inch of runoff from the contributing area. Since they drain down within 24-48 hours, they do not promote mosquito breeding, which have a six to seven day cycle. Residents will be responsible for regular maintenance, which includes weeding and watering during the early years, including trash or debris removal. In succeeding years, maintenance will decrease other than the shredded wood mulch may need replacing as it decays, and thick plants may need separating. The typical pollutants that come with runoff from residential streets, such as fertilizers, oils, metals, etc. are treated through the biological processes in the plants and soils. All of the gardens have a special sediment trap structure that prevents excess sediment from contaminating the garden. The sediment is cleaned out in the spring and fall using the City's Vactor truck (a truck with a high volume vacuum that is also capable of cleaning a multitude of water quality structures throughout the City). The residents pledged to maintain the gardens for a minimum of five years to show the long-term viability and beauty of residential rainwater gardens.